The present invention is directed generally to a wear-resistant refractory composition and more particularly concerns a working refractory lining for insulating and protecting tundish interiors.
In a typical steel casting operation, molten steel is poured from huge ladles into smaller insulated holding vessels known as tundishes. The structure and use of tundishes in casting of steel is well-known to those working in steel industries throughout the world. A useful discussion of tundish design and usage appears in THE MAKING, SHAPING AND TREATING OF STEEL, 9th Edition, 1971, U.S. Steel, Page 710, etc.
Tundishes are designed for containing molten steel in its molten state over a period of time of up to an hour or more during which the steel is fed from the tundishes at a controlled rate to waiting molds. The tundishes are insulated and protected by a safety lining in the form of a matrix of refractory bricks resting on the tundish inner surface which are usually covered in turn by a working lining consisting of a two inch layer of sand and a layer of tundish "top boards". These days, however, refractory veneers are increasingly being used to replace the sand and top board layers.
Tundish top boards are pre-formed shapes made up of a refractory and fiber composition which are placed on a sand intermediate layer over the safety lining of cold tundishes, and keyed or otherwise anchored in place. Top boards cannot be replaced while the tundish is hot and localized wear of the tundish layer requires complete removal and replacement of the working lining. Indeed, tundish top boards are generally good for only a single continuous loading of molten steel due to rapid deterioration of boards and board interstices.
In contrast to traditional top board working linings, refractory veneers may be gunned in place of otherwise applied while the tundish is still hot. An intermediate layer of sand is not required. Furthermore, as localized wear of the veneer occurs, worn portions may be restored without removing the prior coating. Thus, tundish veneer coatings are much more convenient to use than the prior tundish top boards.
However, in order to provide a truly useful and effective alternative to tundish top boards, it is necessary that the tundish veneers be capable of efficiently insulating the tundish interior and withstanding both chemical and physical attack as well as or better than tundish boards. Furthermore, in order to provide a truly practical alternative to tundish top boards, tundish veneers must be made of commonly available and economical materials.
Tundish veneers must have sufficient strength or wear resistance to withstand physical attack by molten steel. The veneers must also exhibit a relatively low thermal conductivity in order to maintain the molten steel within the tundish in its molten state. Although the desired refractory material strength may generally be obtained by developing increasingly dense refractory compositions, unfortunately increasing material density is associated with decreasing thermal insulating properties. Therefore, a successful tundish veneer must exhibit the unusual combination of good strength accompanied by good thermal insulating characteristics.
The tundish working lining, whether it be top boards or refractory veneer, must not only exhibit good thermal insulating properties as well as the ability to withstand physical attack, it also must be capable of resisting chemical attack by the molten steel and accompanying slag. The principal source of chemical attack to the working lining arises from an acid/base reaction between the chemically aggressive slag component of the molten steel and the working lining. Thus, "basic slag" steels cause rapid deterioration of acid-type refractories such as silica and silicate-based refractories and "acid slag" steels cause rapid deterioration of basic-type refractory coatings such as magnesia and calcia based coatings. Since the present compositions are magnesia based, they will not react with basic slag steels and are therefore particularly well suited for use with such steels. An example of an acid resistant refractory veneer appears in U.S. Pat. Application No. 45,531, filed June 4, 1979 and assigned to the assignee of the present application, Nalco Chemical Company.